1. Field of the Invention
The present invention relates to electrochemical energy converters such as fuel cells or electrolysis cells. More particularly, it relates to proton exchange membrane fuel cells (PEMFCs) that include a bipolar separator containing highly porous, electrically conductive, non-metallic material having micro-channels.
2. Description of the Related Art
Existing PEMFCs include a fuel cell assembly. The fuel cell assembly has three basic components: (1) an anode separator, (2) a cathode separator, (3) and a membrane electrode assembly (MEA) interposed between the anode separator and the cathode separator. The MEA consists of an ionomeric membrane with catalyst electrodes adhered to either side; the area of the catalyst is known as the active area. Individually, the anode separator and the cathode separator are unipolar separators; unipolar separators need only the MEA interposed between them to complete the most basic PEMFC. Typically, the anode separator and the cathode separator are placed back to back to form a bipolar separator so that the anode separator and the cathode separator may each interface with MEAs. A fuel cell stack is made by alternating bipolar separators and MEAs, with unipolar separators as terminals of the stack.
PEMFCs typically employ three fluids for proper functioning: a fuel (hydrogen or reformed hydrogen gas), an oxidant (oxygen, enriched air or air), and a coolant (air, water, or specialized coolants). The fuel is transported to one side of the MEA in order to react with the catalyst of the active area. The oxidant is transported to the other side of the MEA in order to complete the electrochemical reaction. A gas diffusion layer (GDL) facilitates the diffusion of both the fuel and the oxidant across the MEA active area. The coolant is circulated through the fuel cell in a designated pattern in order to warm up the fuel cell during initialization and, most importantly, to cool the fuel cell during operation.
Existing PEMFCs suffer from six significant problems: (1) inadequate fluid distribution, (2) incomplete fluid diffusion across the active area of the membrane, (3) electrical current transmission, (4) contact resistance, (5) inconsistent mechanical loading, and (6) insufficient fluid sealing. These problems have precluded the PEMFC industry from mass-producing units of commercial utility. These problems result from the structure of existing bipolar separators.
Existing PEMFCs include bipolar separators that contain macro-channels and macro-manifolds that transport fluids to and from the active area. However, the macro-channels restrict the performance of the fuel cell because they distribute fluids to the active area only along the area that the channels traverse. In addition, the macro-channels and macro-manifolds leave gaps in the volume of the fuel cell that are unable to transmit electrical current, seal the fluids, or support mechanical loading.